Method of etching ethylene diamine tartrate crystals



April 15, 1952 5, EL 2,592,729

METHOD OF ETCHING ETHYLENE DIAMINE TARTRATE CRYSTALS Filed May 25, 19492 SHEETS-SHEET l INVENTOR y E. S. PENNELL A T'TORWE V UNITED staresPatented Apr. 15, 1952 METHODOF ETCHIN G ETHYLENE DIAMINE TARTRATECRYSTALS Everett S. Pennell, New York, N. Y., assignor to Bell'lelephoneLaboratories, Incorporated, New

OFFICE York, N. Y., a corporation of New York Application May 25, 1949,Serial No. 95,216

2 Claims.

control of radio transmitters, and the separar tionof carrier channelsin toll telephony, where sharp frequency discrimination is necessary.

Ethylene diamine tartrate, usually abbreviated as EDT, is one: of thebetter known of the synthetic piezoelectric crystal materials. Largecrystals are growncommercially from small seeds intro? duce'dinto themother liquor, which is a saturated solution of ethylene diaminetartrate. From the large crystals,- blanks are cut approximately tosize, with the prop r orientations and. of the proper shapes to providethe desired characteristics in the finished product. The blanks are thenetched to eliminate surface irregularities and insure high stability ofthe frequency and other constants of the finished plate. Theimmersionetching brings the crystal closer to the proper frequency, butfinal frequency adjustments are commonly made following the plating ofthe etched surfaces with gold or a similar metal to serve as surfaces towhich electrical connections may be made.

A substantial amount of wastage has been experienced in the preparationof EDT plates because of the presence of bloom, which appears on thecrystalline surface as whitish. spots. Bloom interferes with thefunctioning of the crystal, and must be removed. In some cases thecrystal may be then reprocessed for use at higher frequencies than wereoriginally intended; in others, it must be discarded. Bloom has beenidentified as EDT-lHzO, a previously unknown monohydrate form of EDT. Itgrows as a clear, crystalline material, but readily. gives up its waterof crystallization on exposure to dry air, leaving a finely. dividedanhydrous residue. It has been found that monohydrate formation, whichis mosttroublesome in the etching stage, may be eliminated by the use ofthe method and apparatus described below. The etching ,must be carriedout at an elevated temperature, and

the crystals then. dried substantially instantanevously in such a waythat the temperature cannot.

drop below 41 C. at any point on the surface with which water may be in.contact.

The crystals to be etched are mounted in a basket attached to the shaftof a high speed mos tor, and the motor and basket lowered until thecrystals are submerged in the-etching bath. The

vessel holding the etching bath is mounted on a turntable, which isrotated slowly to agitate the etchant. The etchant may suitably be a,mixture of propylene glycol and water.

the required depth of etch has been obtained, the basket is raised untilthecrystals are clear of the bath, and the high speed motor energized sothat the crystal basket and crystals are rotated rapidly, subjectingthem to a radial acceleration ofabout 600 G, to drive off the etchant.The

motor, crystal basket, andetching bath with its turntable, are enclosedin a heat-insulated chamber, the temperature of which is kept above the41 C. minimum by heating and air-circulating means controlled by athermo-regulator.

When the crystalsand basket have been centrifuged. dry, the chamber isopened and the crystals removed. They must thereafter be protected fromwater from any source, including perspiration, breath moisture, or,excessive humidity, during, subsequent steps in production, When theetching is carried out as described,

above, it has been found that the loss from bloom formation may bereduced to a negligible,

amount.

The invention may be better understood by reference to the drawings inwhich:

Fig. 1 is a perspective view of a preferred embodiment of theapparatusof the invention, par- It is to be understood that the embodiments:

shown are illustrative onlyof the principles of the invention, andthatit is intended to include within the scope of the appended claims allvariations in the form and arrangementsof the elements, and allsubstitutions of materials,

which may occur to those skilled inthe art.

In Fig. 1 of the drawings there is: shown a;

After 3 simplified embodiment of the equipment for practicing theinvention. Base I supports a closed, controlled-temperature chamber 2,the walls 4 of which are made of suitable heat-insulating material. Thetop 5 of the chamber 2 is made of a transparent material such as thatknown under the trade name of Lucite, a

thermoplastic methyl methacrylate resin, in order that the attendant mayinspect the operations within the chamber. An observation panel 6 ofsimilar material is slidably disposed in guides 1 in one of the chamberwalls 4, adjacentto the etching bath vessel 9. Panel tmay be raisedbymeans of a knob is to afford access to the vessel 9 when the etchant isto be renewed, or when the etched crystals are to be replaced by a newbatch for treatment.

The etching bath vessel 9 is mounted on a turntable I l, which isrotated at a very slow speed, usually about one revolution per minute,by means such as those shown in detail in Fig. 4. An electric motor l2,acting through a worm gear l4 and a pinion gear l5 provides for thisrotation. The direct connection between pinion gear I5 and turntable His made through a shaft 16 extending through a journal bearing 11 fixedin a bearing support frame IS. The bearing support frame I9 is securedto a subbase 20, which in turn rests on base l. subbase 20 acts also asan inner positioning guidefor the walls 4 of chamber.

' Spring clips 2| are provided on the turntable H and arranged toreceive the vessel 9 and hold it centered on turntable II. Vessel 9 ispreferably transparent, easily cleaned, and chemically inert withrespect to the etchant. A glass beaker, of the type commonly used inchemical laboratories, and of suitable size, is satisfactory for thispurpose.

The etchant, which is preferably a mixture of propyleneglycol and water,is stored prior to use in a closed tank 22, mounted within chamber 2 andfillable through a port 23, until it has warmed up to the ambienttemperature, which is held at 45 C. or above. It is then transferred bygravity feed to the vessel 9 through operation of an externallycontrollable'valve 24. It is necessary to retain the etchant in theclosed tank 22 until it has reached equilibrium with the temperature ofchamber 2, and transfer it to vessel 9 only just before etching, becauseevaporation from the surface of the etchant, once it has been deliveredto the vessel 9, will cool it rapidly. A drop of several degrees belowthe chamber temperature will occur, mostly in the first fifteen minutesof exposure, and more slowly thereafter, so that the time during whichsurface evaporation may occur should be minimized. Fresh supplies ofetchant must be introduced through port 23 to maintain proper strength.

In some cases it may be desirable to use the embodiment shown in Fig. 4,in which the temperature of the etchant in the vessel 9 is separatelycontrolled and the tank 22 may be omitted. A heating coil 25 is disposedaround vessel 9, but spaced therefrom sufficiently to allow the vesselto be readily inserted or removed. Heating current is supplied to coil25 from a conventional source, not shown, through leads 26 under thecontrol of a thermo-regulator 21, so mounted, as by a bracket 28 securedto the centrifuge motor 29, that it is suspended in the liquid duringthe etching. With this construction, the temperature of the etchant maybe held within narrow limits than with that shown in Fig. 1.

Y 4 In either embodiment, centrifuge motor 29 is fixed to a rigidsupport rod 30. Rod 30 extends slidably through the chamber top 5 and issecured by a clamp 3| which may be loosened to permit adjustment of thevertical position of the motor from the outside of chamber 2. Clamp 3|is secured to a relatively heavy main support arm 32 extending upwardlyfrom the subbase 20. Main support arm 32 carries also an alinementbracket 34 through which the support rod 30 is slidably journaled withinchamber 2. Bracket 34 serves to maintain the axial position of motor 29properly fixed, particularly during the centrifuging. Bracket 34 must behigh enough, however, to allow motor 29 to be raised for removal ofvessel 9 without injury to the crystal basket 35, which is fixed to themotor shaft, and is described in detail below. Bracket arm 35 fixed tobracket 34 carries a thermo-regulator 31 which controls the chambertemperature, and which, like regulator 21 shown in Fig. 4, is of aconventional type.

The temperature is maintained substantially uniform throughout chamber 2by provision for rapid forced circulation of the air under the controlof thermo-regulator 31, which may be adjusted as indicated by experienceto maintain thetemperature in all parts of chamber 2 above the desiredminimum.

It has been found advantageous to use a pair of incandescent bulbs a theheating elements, and to enclose them in a turmel which protects thevessel 9 from direct radiation. As seen in Fig. 1, incandescent lampbulbs 38 and '39 are conventionally mounted in a corner of chamber 2,preferably diagonally opposite vessel 9, to secure a relatively eventemperature gradient throughout the chamber. Lamps 38 and 39 areenergized from any convenient power source, not shown; lamp 38 is feddirectly from that source, while the connection to lamp 39 is completedthrough thermo-regulator 31. The size of the lamps is so chosen that thesteady heat from lamp 38 will almost balance the heat loss from chamber2 after it has been brought up to a temperature of about 45 C. Theadditional heat necessary to keep the chamber from falling below thattemperature is then supplied by lamp 39 under control of thethermo-regulator 3B. Tunnel walls 40 and 4| of heat-insulating materialare disposed on subbase 29, cooperating with adjacent chamber walls 4and subbase 20 to form a tunnel 42 open at the upper end only. A smallhigh-speed centrifugal blower unit 44 is mounted on subbase 20 andarranged to maintain continuous rapid circulation throughout the chamber2 by drawing air therefrom and forcing it into the lower part of tunnel42 through a port 45 formed in tunnel wall 49. The air flows around thelamps 38 and 39, is heated, and is discharged back into the chamber fromthe open upper end of tunnel 42.

Certain additional equipment is disposed on.

base I outside the chamber 2. This includes a switch box 46, in whichare mounted the relays,

not visible in the drawings, actuated by thermoregulator 31, and also bythermo-regulator 21 in the embodiment of Fig. 4. These relays controldirectly the supply of current to the lamp 39 and to the heating coil25. A transformer 41 is provided to convert any conventionalalternating-current power supply to meet the needs of the blower 44,which is conveniently a compact type designed for low voltage operation.A manual switch 49 is provided to control the operation of centrifugemotor 29. supplied with power through lead 59 from a conventionalsource, not.

, centrifuge motor 29.

shown. Similarly not shown are theuconven tional power sources supplyinglamp currentto;

switch box lfiflthrough lead blower motor current to transformer ATthrough lead 52, and turntable motor 12 current through lead 54. Itwillbe obvious that a control panel could be provided if desired, on whichcontrol switches for all elements utilizing electric current could bebrought together. Such a showing, as well as the wiring detail, has beenomitted here for the sake of clarity, as itis well within the ability ofanyone skilled in the art to provide such connections.

Returning to the construction of the crystal basket 35, it consists, asseen in'Fig. 4, of three axially spaced discs 6| of Lucite fixed on a.hub 62 secured by a set screw 63. to the shaft 64 of A. plurality ofevenly spaced axially aligned bores 651s formed in the discs 61, withthe ends of each bore carefully chamfered. The bores 63 in the lowermostof discs 6| are of slightly smaller diameter than those in the otherdiscs. This permits a crystal to be dropped into each group of alignedbores with its longitudinal axis parallel'to the shaft of motor 29, andsupported vertically by the lowermost of discs 6| without interferencewith the free circulation of the etchant about the crystal. Durinetching, with relatively slow movement of the etchant relative to thecrystals the alinement of the crystals in the bores is immaterial.During centrifuging the greater magnitude of the centrifugal forcesconstrains the crystals to assume positions in their bores substantiallynormal to radii of the discs El drawn through the crystal centers. Thesmooth bores 65 and 86 with chamfered edges offer negligible resistanceto this change of position; with proper spacing of discs 6 I, adequatesupport for the crystals during centrifuging is provided, and nobreakage results. Obviously, the size of the discs and bores, and thenumber of discs and bores used, may be changed to accommodate differingsizes of crystals.

When a batch of crystals is to be etched, chamber 2 is brought to atemperature of about 45 C., and fresh etchant in the closed tank 22 isallowed to reach equilibrium with the chamber temperature. Verticalsupport rod is lifted to raise the crystal basket above vessel 9,observation panel 6 is opened, and crystals 60 are inserted in thebasket. Panel 6 is quickly closed,

after a short Wait for temperature equilibrium to be reestablished inthe chamber, the proper quantity of etchant is transferred, undercontrol of valve 24, from the storage tank 22 to the vessel 5), toconstitute the etching bath T0. Rod 3G is then lowered until all thecrystals are submerged in the etchant, and the turntable motor i2 isstarted. This position of the crystal basket is seen in Fig. 1.

Etching is continued for the time, which may be determined by meansdiscussed hereafter. Rod 30 is then raised until the crystals 60 areclear of the bath 10, as shown in Fig. 4, and switch 49 thrownimmediately to energize the centrifuge motor 29. The excess solutiondriven off the crystals and basket is caught by the wall of vessel 9 andreturned to the bath Ill. The 3600 R. P. M. motor used subjects thecrystals to a centripetal acceleration of about 600 G., which isadequate to remove all surplus moisture from the crystals and basketsubstantially instantaneously. Rod 30 is then raised until the basket 35clears vessel 9, turntable motor I2 is stopped, panel 6 opened, and thecrystals 80 removed from the basket. In: some. cases it may be desirableto remove basket. 3'5 as a. unitto prevent cooling chamber 2 unduly. Thecrystalsv must be kept dry duringzsubsequent manufacturingstages.

Vessel 9 is then removed to discard the used.

etchant, and replaced in the. chamber 2-. It the embodiment of. Fig. 4'is being used,.a. fresh supply of .etchantis added beforethevessel 9 isreplaced; intheembodiment of Fig; Lit. is added from tank 22afterreplacement ofr thetvessehin the chambers With a new. group: ofcrystals in:- serted, the cycle is: ready to be repeated.

The lengthof submersion in: theetching bath. 1 isdeterminecli byseveral. factors; amongwhich are the etchant used, its temperature and.concentration, and the depth of etch required. It is preferred to use a.mixture of waterand propylene-glycolpas the etchant, with. about. 50Zpercent water by volume.

Propylene glycol-water is more satisfactory for this purpose than othercommonly used mixtures, such as methyl alcohol and water, since lesstemperature shock results in drying from chamber temperature, and sincethe glycol evaporates more slowly from the mixture than does the water,so that changes due to evaporation into the heated chamber atmosphereresult in a less active etchant. This is desirable to prevent excessiveetching, and marking of the etched faces during drying. That it isimportant to control the concentration of water carefully may be seen byinspection of the graph shown in Fig. 2. The curves there shownrepresent the depth of etch in millimeters for a two-minute immersionperiod as a function of temperature, in the range between 35 C. and 45C. It will be observed that, at the chamber temperature of 45, anincrease of only 9 per cent in water content, from 50.0 per cent to 54.5per cent of the mixture by volume, increases the depth of etch from .053millimeter to .117 millimeter, or more than doubles the rate of etching.

The relation between the depth of etch and time of immersion for a 54.5per cent water mixture is shown in Fig. 3. The required depth of etch toobtain approximately the proper crystal size for a given frequency maybe readily obtained by micrometer measurements of the unetched blanks,and the necessary etching time quickly obtained by the use of Fig. 3, oran equivalent graph for etchants of differing strengths. Final precisionadjustment requires electrical measurement of the natural frequency ofoscillation of the crystal, in many cases with the appropriate crystalfaces plated with a metal coating. This final adjustment is commonlymade by grinding, by the addition of metal plating, or by local etchingand drying carried out in accordance wtih the teachings of thisinvention, at a temperature of about 41 C. The adjusting to frequency iswell understood by those skilled in the art, but the present inventionaffords the assurance that the final etching steps will not result inthe destruction of the crystal through monohydrate formation.

It will be seen by inspection of the curves of Figs. 2 and 3 that aswith proper control of temperature of etchant strength, the depth ofetch is related to immersion time in a simple manner. Hence theoperations performed manually as described above may be easilyaccomplished by automatic equipment making the etching process readilyadapted to large scale production.

7 In summary, the method and apparatus described above make it possibleto etch EDT crystals without allowing the formation of the monohydrate,which has constituted a serious;

trate crystal plates, the method of reducing said plates to size byimmersion in an etchant bath of water and material selected from thegroup in- 20 eluding propylene-glycol and methyl alcohol;

which comprises heating said bath and plates to a temperature in excessof 41 C., preventing the temperature from falling below 41 C; at any;

point on said plates during immersion, and drying said platesinstantaneously on removal from said bath.

EVERETT S. PENNELL.

REFERENCES CITED The following references are of record in the file ofthis patent: I

UNITED STATES PATENTS Number Name Date 2,348,631 Keehnel May 9, 19442,416,475 I Friedman Feb. 25, 1947 I 2,416,716 Ross Mar. 4, 19472,446,443

Tibbetts Aug. 3, 1948

1. THE METHOD OF ETCHING ETHYLENE DIAMINE TARTRATE CRYSTALS WHICHCOMPRISES IMMERSING SAID CRYSTALS IN A MIXTURE OF WATER ANDPROPYLENEGLYCOL IN CONTROLLED PROPORTIONS FOR A DESIRED TIME, LIFTINGSAID CRYSTALS FROM SAID MIXTURE, AND INSTANTANEOUSLY DRYING THE SURFACEOF SAID CRYSTALS WHILE MAINTAINING SAID CRYSTAL, AND SAID WATER, ANDPROPYLENE-GLYCOL MIXTURE AT A TEMPERATURE OF AT LEAST 41* C.